Definition of function word in English:

noun

Each strong position in the previous line is occupied by the stressed syllable of a content word; each weak position is occupied by a prosodically-weak function word, or by the unstressed syllable of a polysyllabic word.

Granted, sometimes the repeated function word could be helpful, as in Hacker's example from page 91.

This option is especially likely to present itself in cases where the function word undergoes phonological reduction, or ‘coalesces’ with its host.